This fitness DVD uses cutting-edge science to help you burn more calories per minute than typical workouts. In fact, you'll pack more exercise into 15 minutes than most people do into sessions that last twice that long. So don't think of these workouts as short: Think of them as efficient. Super-efficient. These 15-minute workouts will help you:

Trade Fat for Muscle When you choose your exercises wisely, a handful of moves—just four in some cases—is all you need to change your body composition, according to Prevention fitness advisor and strength-training researcher Wayne Westcott, PhD, CSCS, instructor in the exercise science department at Quincy College in Massachusetts. "Navy research shows you can get tremendous overall improvement—losing 4 pounds of fat and adding 2 pounds of muscle in 8 weeks—by doing just four exercises that work every major muscle," he says.

Burn More Calories The calorie-burning benefits of even the shortest strength-training bout keep coming long after you've left the gym. In a study from Southern Illinois University, researchers found that when volunteers did just one set of nine exercises, or about 11 minutes of strength training, 3 days a week, they increased their resting metabolic rate (the calories burned when just hanging out) and fat burning enough to keep unwanted weight at bay.

Stay Young Unless you do something to stop it, your body loses about half a pound of muscle a year after age 20, says Tina Schmidt-McNulty, exercise specialist at Purdue University Calumet. That may sound nearly insignificant, but when you consider that muscle is your body's biggest calorie burner—burning five times as many calories per pound as fat—it's like "taking your foot off the gas pedal of your metabolism right as you enter adulthood," explains McNulty.